In a conventional condition monitor system, when a certain device fails or does not operate normally, the device is stopped from operating, but allowed to operate under less dangerous condition or reset not to cause abnormality. This system includes a microcomputer and a condition signal generation circuit, which performs predetermined operations such as resetting, when the microcomputer fails to generate periodic pulses due to abnormality. Such a circuit is disclosed in JP 2000-244292, for instance.
In case of an electronic control unit for controlling an electric power supply to various vehicle parts, a condition monitor system is required to avoid uncontrollability of the vehicle parts. Specifically, it is required to maintain the electric power supply to the vehicle parts during vehicle running. It is also required not to shut off the electric power supply irrespective of vehicle running condition as long as a microcomputer for the electric power supply control operates normally.
The vehicle running can be determined based on a time interval of speed pulses successively generated by a vehicle speed sensor. The microcomputer operation condition can be determined based on a time interval of periodic pulses, which is successively generated from the microcomputer. The condition monitor system is preferably constructed to shut off the electric power supply only when both the speed pulses and the periodic pulses are not generated.
The time interval of pulses may be measured by charging and discharging a capacitor, because a charge voltage of the capacitor changes with the length of the interval. The intervals of the speed pulses and the periodic pulses differ largely, and hence the capacitor and its charge/discharge circuit must be provided for each of the speed pulse and the periodic pulse. As a result, the condition monitor system requires many circuit components and becomes large in size.